It's simple, string. There is no convert method in this class. If you need type conversion, you can convert numbers into strings (definitely), just use ".". After the toString () ". If you need to convert a string into a number, you can use, for example, int. Parse (parameter) and double. Parse (parameter). But you need to pay attention to whether it can be converted, otherwise it will be abnormal. If you are not sure, you can put the whole sentence block in try{}.